Data from: Persistence, impacts and environmental drivers of covert infections in invertebrate hosts

Background: Persistent covert infections of the myxozoan, Tetracapsuloides
bryosalmonae, in primary invertebrate hosts (the freshwater bryozoan,
Fredericella sultana) have been proposed to represent a reservoir for
proliferative kidney disease in secondary fish hosts. However, we have
limited understanding of how covert infections persist and vary in
bryozoan populations over time and space and how they may impact these
populations. In addition, previous studies have likely underestimated
covert infection prevalence. To improve our understanding of the dynamics,
impacts and implications of covert infections we employed a highly
sensitive polymerase chain reaction (PCR) assay and undertook the first
investigation of covert infections in the field over an annual period by
sampling bryozoans every 45 days from three populations within each of
three rivers. Results: Covert infections persisted throughout the year and
prevalence varied within and between rivers, but were often > 50%.
Variation in temperature and water chemistry were linked with changes in
prevalence in a manner consistent with the maintenance of covert
infections during periods of low productivity and thus poor growth
conditions for both bryozoans and T. bryosalmonae. The presence and
increased severity of covert infections reduced host growth but only when
bryozoans were also investing in the production of overwintering
propagules (statoblasts). However, because statoblast production is
transitory, this effect is unlikely to greatly impact the capacity of
bryozoan populations to act as persistent sources of infections and hence
potential disease outbreaks in farmed and wild fish populations.
Conclusions: We demonstrate that covert infections are widespread and
persist over space and time in bryozoan populations. To our knowledge,
this is the first long-term study of covert infections in a field setting.
Review of the results of this and previous studies enables us to identify
key questions related to the ecology and evolution of covert infection
strategies and associated host-parasite interactions.